This invention relates to a mechanically preloaded packing assembly and, deals more particularly with a precision mechanically preloaded packing assembly utilizing Belleville springs or spring washers to energize a packing ring in a high pressure gate valve to provide an adequate seal between the stem and appropriate valve parts at both high and low pressures.
Heretofore, valves have been called upon to handle increasingly higher pressures, particularly valves which are used in the oil and gas industry where the trend has been toward deeper wells in which pressures up to 30,000 psi and above are encountered. Valves must also contend with noxious substances such as hydrogen sulfide gas which can pose serious environmental dangers if permitted to leak. Furthermore, the valves are often exposed to high temperature fluids which can cause thermal distortion of the valve and the various components thereof, including the packing assemblies.
The various types of packing materials and valve stem packing arrangements that have been proposed or used in the past for high pressure applications or applications where the valve is subjected to handling wide pressure variations or harmful noxious or toxic substances have not been entirely satisfactory. Such a high pressure packing assembly is shown in U.S. Pat. No. 4,262,690 to Binegar which utilizes a plurality of plastic packing retainer rings which are then urged together by fluid pressure to squeeze the packing elements firmly against the stem and corresponding body part to seal therebetween.
However, in the assembly of valves, it is frequently important that the packing assemblies be placed in a preloaded condition prior to use in order that a seal can be established at low fluid pressures as the pressure builds as well as at high pressures. In mechanically preloaded packing assemblies which utilize Belleville springs or spring washers to provide the preload tolerance stack up resulting from manufacturing variances of the metal parts of the valve presents a problem because a varying number of such springs must be employed depending on the degree of manufacturing variance present in the particular valve body. The determination of the proper number of such springs is a tedious and time consuming process and when performed by field personnel can lead to hazardous errors. Oftentimes it is necessary to stack different sets of Belleville springs 180 degrees with respect to one another within a single packing assembly so that a greater spring washer deflection can be obtained while maintaining the same axial load acting on the packing elements. However, such multiple use of spring washers makes it extremely difficult to determine the precise preload exerted on the packing elements due to the difficulty in ascertaining the exact deflection of the spring washers upon energization of the packing assembly. Also spring washers can be over deflected beyond the elasticity of the metal parts which will then destroy the preloading capacity of the washers. Furthermore, such non-uniformity and multiplicity of spring washers adds to the manufacturing costs of the valve.